Much evidence indicates that neoplastic transformation of cells involves changes in the molecules composing cell surfaces. Further, some of the altered behavioral characteristics of neoplastic cells can be related to such changes in their surface molecules. The objective in this study is to identify and to determine the spatial relationship of molecules and reactive groups in plasma membranes of normal and neoplastic cells using immunoelectron microscopy. Initially lymphocytes from normal human volunteers, patients with chronic lymphatic leukemia, and patients with acute lymphatic leukemia will be studied. Antibodies to blood group A, HL-A7 and Ig antigens will be labelled with either ferritin or dextran-250. Concanavalin A binding sites will also be studied. By using two different labels, ferritin and dextran-250, two groups can be identified on the same cells, i.e., Ig determinants and Con A receptors, and blood group A antigens, HL-A7 antigen and blood group A antigen, etc. These labelled ligands will be used to compare normal and neoplastic cells. Freeze etch and thin section preparations will be examined by EM to determine the surface distribution of these molecules.